1. Field of the Invention
The present invention relates to a microcomputer for controlling an ultrasonic motor and to a method for controlling the ultrasonic motor.
2. Description of Related Art
There is a known ultrasonic motor having a stator that has a piezoelectric body and a moving body that performs rotational movement. In the ultrasonic motor, a traveling wave is generated on a stator which is in turn excited by applying a control signal having a frequency in an ultrasonic frequency band (20 kHz or higher), to the piezoelectric body. The ultrasonic motor transmits mechanical energy generated by elliptic movement that occurs at respective points on the excited stator to the moving body to rotate the moving body so as to produce power.
The ultrasonic motor controls the rotation speed of the moving body by using the frequency of the control signal applied to the piezoelectric body. However, ultrasonic motors largely vary from one another in their characteristics. Accordingly, the rotation speed obtained at a particular frequency varies among the ultrasonic motors, and the ultrasonic motors, therefore, need to be controlled individually. The technique shown below is disclosed as a technique of controlling the ultrasonic motor.
JP-A-2003-153558 discloses an oscillatory wave motor drive control device that applies an alternative sine wave with low distortion to the piezoelectric body so that partial wear-and-tear (i.e., damage) less likely to occur on the stator, and an oscillatory wave motor having a longer service life can be realized. The oscillatory wave motor drive control device of JP-A-2003-153558 is a drive control device for an oscillatory wave motor that excites an electro-mechanical energy conversion element to obtain a driving force by applying a frequency signal to the electro-mechanical energy conversion element, which includes: an encoder for detecting a working speed of the oscillatory wave motor; a speed difference detecting unit for outputting a speed difference signal by calculating a speed difference between the working speed detected by the encoder and a target speed of the oscillatory wave motor; a frequency setting unit for setting a frequency increase/decrease amount based on the speed difference signal; a reference voltage unit for generating a reference voltage corresponding to the target speed; a voltage detecting unit for detecting a voltage of the frequency signal applied to the oscillatory wave motor; a comparing unit for outputting a voltage difference signal by comparing the reference voltage generated by the reference voltage unit and the voltage detected by the voltage detecting unit; a sine wave transmitting unit for transmitting a sine wave signal whose frequency is obtained by calculating the frequency corresponding to the target speed and finely adjusting the frequency based on the frequency increase/decrease amount set by the frequency setting unit and whose amplitude is determined based on the voltage difference signal output from the comparing unit; a driving signal generator for generating a plurality of sine wave driving signals of different phases based on the sine wave signal output from the sine wave transmitting unit; and a motor driving circuit for applying periodic signals to the oscillatory wave motor based on the plurality of sine wave driving signals output from the driving signal generator.